Estrogen interaction with the immature rat uterus will be studied as a model system of hormonal regulation of the reproductive tract and tissue growth. Receptor proteins for estrogens in the cytoplasm and nucleus of the uterus will be compared to determine the nature of the transformation. Attempts will be made to isolate the receptors using affinity column procedures. Pilot studies suggest that purification of the induced protein of the uterus is feasible. After purification, characterization of the physical properties may give some clues to its biological function.